Copper and copper base alloys



Patented May 21, 1940 UNITED STATES.

PATENT OFFICE COPPER AND COPPER- BASE ALLOYS No Drawing. Application April 13, 1938, Serial No. 201,725

1 Claim.

The invention relates to copper and copper base alloys in which the matrix or continuous phase is an alloy of the alpha copper type, such as an alpha brass, an alpha bronze, a cupronickel, or a high nickel brass.

Copper and copper alloys of the alpha type are ordinarily cast and then heat treated and wrought in the following fashion. The metal when cast does not have a homogeneous structure, but contains some portions which have a copper content lower than the average. Hence, the metal is usually homogenized by heating at an elevated temperature below 850 C. Mechanical working is generally efiected while the metal is cold. Because these metals harden when they are cold worked, if the total amount of mechanical workings is substantial it is done in several working steps with intermediate annealing steps at a temperature between 300 and 850 C., usually between 480" and 660 C.

Such metals have a strong tendency for rapid grain growth at elevated temperatures above 300 C. Grain growth during the annealing or homogenizing steps adversely affects the strength and ductility of the metal and its rolling, drawing, and other working properties. Furthermore, the amount of cold work done on a given section has a marked effect on the rate of grain growth, and

annealing will usually produce grains of differ- 3o ent and unpredictable sizes in different parts of the same article. A coarse grained material has a greater tendency than a fine-grained material to split, tear and roughen.

It is the primary object of the invention to improve the rolling,,drawing, and other physical properties of copper and alpha alloys of copper by inhibiting grain growth at annealing temperatures.

I have found that the presence of a relatively small amount, between 0.05% and 0.5%, of finely divided precipitated chromium in copper and copper alloys of the alpha type effectively inhibits grain growth in this material at temperatures up to about 850 C. After heating at annealing temperatures below 850 C. the grain size of the chromium-containing material is only about 10% to of that of material containing no chromium but otherwise of similar composition. The addition of chromium provides a uniformly fine grain throughout the metal, regardless of wide differences in the amount of cold work done on the metal and regardless of .the specific annealing temperature and time used. Thus, conditions of the cold working and annealing operations need not be limited, as they ordinarily are when treating alpha brasses, by the necessity to control grain growth. Because the grain size changes but slightly over a wide temperature range, drawing conditions and grain size can be readily controlled. 5

For most purposes, 0.2% chromium is sufficient to impart the full benefits of the invention, and in no instance should enough chromium be added to aflect materially the properties of the copper base metal in any respect other than those de- 10 rived from the inhibition of grain growth.

The chromium may be added as chromium metal or ferrochromium or, preferably, cuprochromium, to themolten copper base material to be treated, and the melt then cast. The chro- 15 mium is precipitated by maintaining the alloy within the temperature range of 350 C. to 700 C. for a time, suitably .a few minutes at the higher temperatures within such range or several hours at the lower temperatures. This step 20 may coincide with the customary homogenizing anneal. Thereafter, the material may be mechanically wrought and annealed in conventional manner, but the alloy must not be permitted to reach temperatures above 850 C., because at such elevated temperatures the chromium will rapidly dissolve and permit grain growth. There is some tendency for the chromium to dissolve at temperatures between 700 and 850 C., but the rate of solution at such temperatures is rela- 3o tively slow.

It has been proposed previously'to add chromium to copper and copper base alloys, but all of such proposals have been for the purpose of producing a hardenable alloy. According to 85 these proposals, chromium is added, usually in an amount greater than 0.5%, to copper or a copper base alloy, and such chromium is then fully dissolved by heating the alloy at an elevated temperature above 850 C. for a short time or at a temperature between 700 C. and 850 C. for a very long time. As a general rule, silicon is also added to intensify the hardenability. The alloy containing chromium wholly in solid solution is then mechanically wrought and annealed, the annealing being carried out at temperatures high enough to keep the chromium in solution. After being wrought, the alloy is hardened by heating within the temperature range of 350 C.

to 700 C. to precipitate the chromium. In such processes, the chromium, since it is in solid solution during the annealing steps, is ineffective to inhibit grain growth. According to the present invention, to the contrary, the chromium is precipitated before mechanical working is comg menced and it is thereafter kept in uniformly distributed, finely precipitated form throughout the working and annealing steps.

This application is in part a continuation of my application Serial No. 64,884, filed February I claim:

In the manufacture of wrought articles composed of alpha brass containing between 0.05% and about 0.2% chromium, wherein the molten alloy is cast, mechanically wrought, and annealed, the method of preserving a fine-grained structure and inhibiting grain growth during annealing which comprises precipitating the chromium in a uniformly distributed finely divided form by heating the alloy at a temperature between 350 C. and 700 C. and thereafter repeatedly and alternately mechanically working and annealing the alloy at temperatures below 350 C., the chromium being maintained in precipitated form during every step of the process subsequent to the said precipitating step by keepring the alloy at a temperature below 850 (3., and the amount of precipitated chromium being insufiicient to raise materially the hardness of said alpha brass or to impart substantial capacity to harden by heat treatment.

MICHAEL G. CORSON. 

